Factors involved in the formation and elimination of neuromuscular synapses will be studied in two different preparations. The first consists of isolated adult skeletal muscle fibers maintained in vitro with embryonic chick neurons (ciliary ganglion). Differences in synapses formed at end plate and extrajunctional regions will be an area of special attention. In addition, factors involved in acetylcholine (ACh) receptors will be studied, namely: ACh receptor synthesis, mobility, and (by noise analysis) channel open-time of "perijunctional receptors." The second preparation is a rat muscle (studied in vivo) which receives its motor innervation from two separate peripheral nerves. The proposed study involves the effect of nerve activity in the elimination of polyneuronal innervation.